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Weak Reflections (weak + reflection)

Distribution by Scientific Domains

Chemistry	100%

Selected Abstracts

Ab initio structure solution by iterative phase-retrieval methods: performance tests on charge flipping and low-density elimination

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2010
Frank Fleischer
Comprehensive tests on the density-modification methods charge flipping [Oszlányi & Süt, (2004). Acta Cryst. A60, 134,141] and low-density elimination [Shiono & Woolfson (1992). Acta Cryst. A48, 451,456] for solving crystal structures are performed on simulated diffraction data of periodic structures and quasicrystals. A novel model-independent figure of merit, which characterizes the reliability of the retrieved phase of each reflection, is introduced and tested. The results of the performance tests show that the quality of the phase retrieval highly depends on the presence or absence of an inversion center and on the algorithm used for solving the structure. Charge flipping has a higher success rate for solving structures, while low-density elimination leads to a higher accuracy in phase retrieval. The best results can be obtained by combining the two methods, i.e. by solving a structure with charge flipping followed by a few cycles of low-density elimination. It is shown that these additional cycles dramatically improve the phases not only of the weak reflections but also of the strong ones. The results can be improved further by averaging the results of several runs and by applying a correction term that compensates for a reduction of the structure-factor amplitudes by averaging of inconsistently observed reflections. It is further shown that in most cases the retrieved phases converge to the best solution obtainable with a given method. [source]

Ab initio structure solution by charge flipping.

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 1 2005

The original charge flipping algorithm [Oszlnyi & St (2004). Acta Cryst. A60,34141] is an amazingly simple structure solution method which works ab initio on high-resolution X-ray diffraction data. In this paper, a new version of the algorithm is presented that complements the phase exploration in reciprocal space. Instead of prescribing observed moduli of all structure factors, weak reflections are treated separately. For these reflections, calculated moduli are accepted unchanged and calculated phases are shifted by a constant ,, = ,/2. This means that the observed data of weak reflections are not used in the iteration, except for the knowledge that they are indeed weak. The improvement is drastic, in some cases the success rate is increased by a factor of ten, in other cases a previously unsolvable structure becomes solvable by the modified algorithm. [source]

Superspace description of the modulated structure of the metal-salt-hybrid Bi7,,,,Ni2Br5,,,2, (, = 1/9)

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2009
B. Wahl
The compound Bi7,,,,Ni2Br5,,,2, = (Bi3Ni)2[Bi1,,,,Br4]Br1,,2, (, = 1/9) is a sub-bromide of the intermetallic phase Bi3Ni. Its crystal structure contains metallic rods, [Bi3Ni], which are embedded in a salt-like matrix of bromido-bismuthate(III) and bromide anions. The non-stoichiometry originates from the variation of the number n of trans edge-sharing octahedra in the [BinBr4n,+,2](n,+,2), oligomers (3 ,n, 5), as well as from vacancies on the sites of the isolated Br atoms. The simplified structure is described in the orthorhombic space group Cmcm with a = 4.0660,(4), b = 23.305,(3), c = 17.130,(2),Å. It shows a statistical distribution of vacancies and orientational disorder of the concatenated octahedra. By choosing the modulation vector q = a*/9 + b*/2, the additional weak reflections of the diffraction pattern can be indexed. In the [3,+,1]-dimensional superspace group Pmnm(,½0)000, an ordered structure model is achieved. The modulated crystal structure bears a strong resemblance to the somewhat higher oxidized sub-bromide Bi7,,,,Ni2Br5 (, = 1/9). [source]

Structure of the defect perovskite Ce1/3NbO3: a redetermination by electron and neutron powder diffraction

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2000
C. Bridges
The crystal structure of the defect perovskite Ce1/3NbO3, cerium niobium oxide, has been re-examined by neutron powder and electron diffraction. The results of a powder neutron Rietveld refinement indicate that the structure is monoclinic: space group P2/m with Z = 4, a = 5.5267,(3), b = 7.8824,(2), c = 5.5245,(3),Å, , = 90.294,(1)°, V = 240.67,(2),Å3 at 298,K with ,2 = 2.570. Previous reports have described the Ce1/3NbO3 structure in a smaller (V/2) orthorhombic cell based solely upon X-ray powder diffraction data. The presence of weak reflections in the electron diffraction pattern provides conclusive evidence for a monoclinic superstructure of the orthorhombic cell. While these superlattice reflections are barely detectable with X-ray radiation, they are clearly visible in the neutron diffraction experiments. The superlattice reflections are shown to arise from a tilting of the NbO6 octahedra which results in the reduction of symmetry from orthorhombic to monoclinic. It is also found that the Ce3+ and Nb5+ cations are displaced from the centres of their respective polyhedra to accommodate the bond-valence requirements of the crystal structure. It is likely that distortions of this type are present in other Ln1/3NbO3 and Ln1/3TaO3 defect perovskites. [source]

Autoindexing the diffraction patterns from crystals with a pseudotranslation

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 6 2009
Nicholas K. Sauter
Rotation photographs can be readily indexed if enough candidate Bragg spots are identified to properly sample the reciprocal lattice. However, while automatic indexing algorithms are widely used for macromolecular data processing, they can produce incorrect results in special situations where a subset of Bragg spots is systematically overlooked. This is a potential outcome in cases where a noncrystallographic translational symmetry operator closely mimics an exact crystallographic translation. In these cases, a visual inspection of the diffraction image will reveal alternating strong and weak reflections. However, reliable detection of the weak-intensity reflections by software requires a systematic search for a diffraction signal targeted at specific reciprocal-space locations calculated a priori by considering all possible pseudotranslations. Care must be exercised to distinguish between true lattice diffraction and spurious signals contributed by neighboring overlapping Bragg spots, non-Bragg diffraction and noise. Such procedures have been implemented within the autoindexing program LABELIT and applied to known cases from publicly available data sets. Routine use of this type of signal search adds only a few seconds to the typical run time for autoindexing. The program can be downloaded from http://cci.lbl.gov/labelit. [source]